There are two principal groups of the plasma sources: capacitively-coupled plasma sources, which utilize RF electric field coupling to the plasma, and inductively coupled plasma (ICP) sources, which utilize RF magnetic field coupling to the plasma.
Capacitively-coupled plasma sources include “planar diode” or “parallel plate” systems, in which a surface to be etched is placed on one electrode that is coupled to an RF generator through a blocking capacitor, which does not allow real current to flow from the electrode to the RF generator. This forces the plasma to find a condition in which the electron current reaching the electrode from the plasma exactly balances the ion current averaged over one RF cycle. Since electrons are more mobile than ions, the electrode acquires a negative potential to limit the electron current and encourage the positive ion current. This negative potential is called the self-biased voltage, which results in an energetic ion bombardment of the surface being etched. Changing applied RF power can control the ion bombardment. A planar triode capacitively-coupled plasma system with two RF electrodes and grounded walls represents another geometry.
Capacitively-coupled sources have widespread use in semiconductor plasma processing, but have some problems and limitations. For instance, the electrical behavior of a capacitively-coupled discharge is influenced by the reactor geometry, where plasma potential depends strongly on the area of the powered electrode relative to the area of all other surfaces in contact with the discharge. In symmetrical systems, both electrodes can be subjected to high-energy ion bombardment, including energetic ion bombardment of walls and fixtures causing sputtering of these surfaces and chamber contamination. The pressures typically used have to be maintained high enough (>100 mTorr) so that sputtering of the grounded surfaces is not a problem. In asymmetric low pressure systems, only the biased electrode is sputtered, as the walls are exposed to low energy ions, and a nonuniform plasma density that peaks in the center is produced. Non-uniform plasma caused by large losses of electrons and ions to the chamber walls causes nonuniform etch rates to result. Optimal geometry at low pressure has been cylindrical geometry, including that in hexode systems, which is not adaptable to 300 mm single wafer semiconductor manufacturing.
A common drawback of the above-described systems is an inability to independently control ion energy and ion flux at a fixed pressure and RF frequency. Planar triode systems solve this problem but require relatively high pressure to eliminate sputtering of the top electrode, which voltage can be reduced by increasing frequency at the top electrode and decreasing frequency at the wafer electrode. In the VHF frequency range, high plasma densities can be generated with low applied voltages. Higher frequencies result in less damage, more uniformity across the electrode, and the ability to process larger substrate areas at more uniform rates. For example, recent developments in capacitively coupled plasma systems step in the direction of higher excitation frequencies in the 30 to 300 MHz range, where high plasma densities can be generated with low applied voltages. This produces a lower damage process with improved uniformity across the electrode, so that larger areas may be processed at more uniform rates.
Inductively coupled plasma (ICP) sources provide relatively low ion energy bombardment and reasonable etch rates. Common ICP sources include coils having planar, cylindrical or dome-shaped geometries. The so-called helical resonator is a cylindrical ICP source in which a movable tap on the coil is used to optimize tuning and power transfer into the plasma. The helicon source is an ICP source that uses an antenna with specific geometry to launch a wave along an externally applied magnetic field, which can couple energy into the plasma electrons. High frequency (2.45 GHz) electromagnetic radiation is also used to generate high-density plasma in the microwave range but excitation is limited to <1011 electrons per cm3, so it is mostly used as downstream plasma for wafer processing. In combination with large magnetic fields (875 Gauss for excitation at 2.45 GHz) an electron cyclotron resonance (ECR) can be achieved with which plasma densities in the 1013 cm−3 range. There are many commercially available plasma sources that can be used to generate high-density plasmas (>1011 electrons cm−3) at relatively low pressures (<10 mTorr) without requiring the application of high voltages.
Plasma sources are used in combination with a capacitively-coupled, RF-powered electrode on which the processed wafer is placed. Such an electrode is often an electrostatic chuck (ESC), which consists of a base plate having cooling channels or heating structure inside and connections for backside gas, DC chucking electrodes, and temperature detectors, etc. In some cases a flexible bellows mounted between a robust backside flange and a bottom chamber flange allows vertical movement of the ESC to enhance process control variables.
In systems with independently RF biased substrate holders, independent control of the ion energy and ion flux can be obtained. Typically, the power used in the high-density plasma source is much larger than the bias power applied to the wafer-bearing electrode. Increasing the bias power increases the ion energy without changing the ion current density. Increasing the source power results in both an increase in the ion flux and a decrease in the ion energy.
Etch uniformity at the wafer is affected by ion flux and ion energy. Generally, the ion flux towards the wafer is a function of plasma density distribution. To achieve uniform etching, uniformly distributed plasma parameters have to be provided. Typically, the TCP sources with spiral coils produce a plasma distribution with a peak at its center at the pressures that are typically used. Multiple coil configurations, magnetic fields, wafer pedestal size and material, and additional chamber hardware are used to improve plasma density distribution. For instance, multipolar magnetic field confinement of a plasma helps to increase plasma density and improve plasma homogeneity, but only for pressures well below 1 milliTorr (mTorr). From these pressures, plasma homogeneity degrades rapidly as the pressure is increased. Another solution has been to provide a magnetic confinement ring around an electrostatic chuck to confine the plasma within the area defined by the ring. Unfortunately, the magnetic confinement ring often produces the well-known cusp effect on the peripheral surface of the wafer due to the magnetic field of the confinement ring.
In ICP systems, the use of dual coils or dual zone coil configurations within an ICP source can partially improve the plasma density distribution. However, because of the distance of the coil from the wafer pedestal, increased RF power is required to compensate radial power loss, for which an additional RF generator and matching unit are typically needed. Still, the sidewall effect is not completely removed from the wafer. Significant chamber modifications may be required. A larger size dielectric windows may be needed, and the window thick enough to withstand atmospheric forces. Additional controlling units and cooling may be required. For current 300 mm size wafers, all these components represent large and expensive consumable parts, increased complexity, resulting in high cost of operation and significantly high overall cost of the machine.
Furthermore, producing uniformly distributed plasma over a wafer may have the undesirable effect of reducing plasma density. A wafer exposed to the reduced plasma density generally takes more time to produce a desired etch or deposition than a wafer subject to a higher plasma density. Hence, the etch or deposition process may take longer to complete in a uniformly distributed plasma environment.
Accordingly, there is a need for an ICP source that produces a high density uniform plasma that is simple and low in cost.